A decrease in renal blood flow, especially if severe and prolonged, may lead to acute renal failure. The factors contributing to renal insufficiency are poorly understood. Severe renal ischemia will be produced in anesthetized rats by temporary complete renal artery occlusion for one hour. Kidney function will be studied before occlusion, immediately after release of the occlusion, and one day later. Glomerular filtration rate will be measured in single nephrons by micropuncture. Kidney capillary blood flow in the superficial cortex and inner medulla (papilla) will be studied by microcinematography. Whole kidney blood flow will be estimated with an electromagnetic flow meter. Pressures in kidney capillaries will be estimated with a servo null device. These studies should elucidate the hemodynamic condition of kidneys during one type of acute kidney failure, and should contribute to our understanding of acute renal insufficiency.